The present invention relates to a residual capacity detecting apparatus which is preferably employed for detecting the residual capacity of a battery installed on an electric automotive vehicle, and a related method for measuring the residual battery capacity.
Conventionally, an electric automotive vehicle has a battery equipped with a residual capacity detector. For example, when a lead battery is installed on an automotive vehicle, the initial discharge characteristics of this lead battery is stored in a memory as table data beforehand. The initial discharge characteristics represents a relationship between a discharge voltage and a corresponding residual capacity of the lead battery in a green state. A terminal voltage of the battery is measured during a discharge operation. Then, referring to the table data stored in the memory, a momentary residual capacity of the battery is calculated based on the measured discharge voltage.
Besides the lead batteries, alkaline batteries have been recently used as electric power sources for electric automotive vehicles because of their large energy densities. Ni--MH batteries and Ni--Cd batteries are representative alkaline batteries. Like lead batteries, the alkaline batteries are secondary batteries which can be used repetitively by recharging.
However, alkaline batteries cause a so-called memory effect (i.e., memory phenomenon) as a result of repetitive charge and discharge operations unless they are fully discharged until the residual capacity is completely reduced to a predetermined discharge terminal voltage. Generally, the discharge characteristics of a battery is represented by a relationship between its discharge voltage and a corresponding residual capacity. For example, the discharge characteristic obtained in a green state of the battery is referred to as initial discharge characteristics.
The problem arising with an alkaline battery causing the memory effect is that its discharge characteristics momentarily varies in accordance with the memory effect. Thus, the discharge characteristics of a used battery possibly differs from its original (i.e., initial) discharge characteristics.
Accordingly, an error may be caused in a calculation of the residual battery capacity when the memory effect of the battery is neglected and the residual capacity is calculated based solely on the pre-stored initial discharge characteristics.
In this respect, Unexamined Japanese patent application No. 7-55903 proposes a residual capacity measuring method for a battery, according to which a discharge amount is calculated by integrating momentary values of the discharge current during a discharging operation. Then, a residual battery capacity is calculated by subtracting the calculated discharge amount from a predetermined full charge capacity (corresponding to 100% residual capacity).
However, according to this conventional art, the following problems will arise.
An electric automotive vehicle consumes electric energy stored in a battery to travel. Accordingly, it is preferable to calculate the residual capacity of a Ni--MH battery in terms of energy (Wh), i.e., using three factors of voltage (V), current (A) and time (h).
However, the above-described conventional art obtains the residual battery capacity in terms of the units of Ah which is not relevant to the energy (Wh).
In a system using a battery, the electric power is generally supplied from the battery. When a battery is subjected to the memory effect, there is a tendency that its discharge current becomes large in response to its discharge voltage lowered due to the memory effect. Accordingly, the battery causing the memory effect tends to discharge earlier compared with a battery free from the memory effect.
Furthermore, it is inevitable that a battery is deteriorated during a long time usage. Thus, an actual full charge capacity of a used battery will be lowered compared with a predetermined full charge amount (i.e., a rated value). In such a case, the battery may have no available capacity irrespective of an indication of a significant amount of residual capacity.